A separator is the device dividing an electrolysis cell into two parts. The term "separator" thus includes a central membrane or diaphragm. In general terms, a separator is a sheet like member which divides the electrolysis cell into two compartments. One is labeled the anode compartment filled with the anolyte. Moreover, the other is the cathode compartment where the cathode is located. In such an arrangement, it is desirable that transport across the separator enables electrochemical conversion to occur. An electrolysis cell is ordinarily used to convert the feed into different compounds or to convert compounds into elements. This exchange is normally carried out by ionic movement across the separator. To this end, the separator must be able to transport ions across the separator from one side to the other. These reactions normally occur in the presence of water which must wet the separator to enable ionic movement.
A classic electrolysis conversion is in the chlor-alkali cell. NaCl and water is supplied to one side. The sodium ions are transported across the separator. This forms NaOH on the cathode side of the cell. Moreover, it liberates chlorine and hydrogen gas from the two electrodes in the cell. The process cannot occur except that ions and associated water move through the separator. In this example, the cathode liberates gaseous hydrogen. This changes the pH of the catholyte by forming OH.sup.- ions on the cathode side of the cell. They react the sodium ions with OH.sup.- to thereby form a strong caustic solution which is recovered as a valuable product.
A high concentration of OH.sup.- ions is observed in the catholyte. While they are formed on the cathode side and are typically discharged in the caustic solution, there remains the possibility that a small percent of the OH.sup.- ions will migrate back across the separator toward and into the anodic chamber. Thus, the separator is called upon to pass positive sodium ions from anode to cathode side while rejecting OH.sup.- ions attempting to migrate in the opposite direction. This is true even where the separator is in ion exchange resin. The hydroxyl back migration is highly undesirable. One showing of high concentration is the output of hypochlorite. This is undesirable. Thus, it is desirable that the separator transfer freely selected ions in the proper direction and forbid specific ion movement in the opposite direction. This enables a more pure product to be made, and it also improves cell efficiency.
It is possible to strengthen the separator by making it thicker or to otherwise impede ionic movement. When this is done, cell efficiency is ordinarily decreased. There are several patents of deNora and perhaps U.S. Pat. No. 4,381,979 is the most typical of the deNora references. As described in deNora, there is a membrane 105 immediately adjacent to a metal screen or mat 113. It assists in holding the screen in location against the separator. It is described in the disclosure as a compressible electroconductive wire mat. Moreover, it is said to have an open mesh knitted wire construction.
The present apparatus is useful with a separator arranged between a widely spaced pair of electrodes on opposite sides of the separator, or those which are immediately adjacent to the separator as in a cell having zero gap between the electrodes. It also operates well with a ion exchanqe resin. An example is a fluoropolymer layer having sulfonic or a sulfonic/carboxylic complex. The present apparatus cooperates also with a diaphragm which relies primarily on a tortious path for ionic movement restriction. The present apparatus contemplates the addition of an electrostatic repulsive charge electrode within the separator. The form is preferably an insulated screen wire embedded in the separator. An alternate form can be obtained by placing a layer of conductive materials in particulate form which is bonded by typical fluoropolymer or even electrically conducting polymer which is porous and placed on the cathode. In any case, the repulsive negative charge repels OH.sup.- ions and thereby enhances the tortuosity effect of the pathway through the membrane. An important aspect of the present invention is the advantage of producing stronger catholyte. That is, there is no loss of OH.sup.- ions across the separator and greater efficiency is thereby achieved in making a stronger caustic solution. Likewise, the strength of the anolyte is increased because it is not subject to degradation of the OH.sup.- ions migrating through the separator. In addition, the invention helps to reduce the concentration of hypochlorite (ClO.sup.-) in the anolyte.
In summary, the present apparatus is thus described as a separator which is either a diaphragm or other ion exchange resin membrane having the added feature of a central coterminous layer. This layer in the preferred embodiment is formed of a fine wire woven into a screen covered with an insulated material. It is connected with a suitable voltage souroe so that it is oharged. It creates a repellent electric (biased negative) field which rejects OH.sup.- to prevent back migration. An alternate embodiment can be made by utilizing finely divided particulate conductor metals bonded with a suitable bonding agent whereby the layer functions to impose a static eleotric field repelling OH.sup.- ions. The repellent action is accomplished to thereby selectively prevent back migration of OH.sup.- ions.